Anaerobic treatment of industrial wastewater

ABSTRACT

Feed water is processed in an anaerobic digester. A solid-liquid separation device, for example a sludge screw thickener, treats a stream drawn from the digester in a recirculation loop. The solids portion is returned to the digester to increase the solids retention time and the TSS concentration in the digester. A liquid portion with less than 5% of the solids in the stream is removed and optionally treated further. The flow rate to the solid-liquid separation device is preferably greater than the influent flow rate. The solid-liquid separation device may receive digestate at a TSS concentration of 4% or more and return a solids portion having a TSS concentration of over 10%. The feed water is preferably one or more industrial waste streams having a COD concentration of 20,000 to 50,000 mg/L and a TSS concentration from 1-5%. The organic loading rate may be 10-12 kg/COD/m3/day.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/636,000 filed Apr. 20, 2102; U.S. Provisional Application No.61/725,842 filed Nov. 13, 2012; U.S. application Ser. No. 13/826,410filed Mar. 14, 2013; U.S. application Ser. No. 13/832,258 filed Mar. 15,2013; and International Application Number PCT/CA2013/050297 filed onApr. 18, 2013 which are hereby incorporated by reference.

FIELD

This specification relates to a process and apparatus for treating wasteproducts, such as industrial wastewater or industrial solid waste,involving anaerobic digestion.

BACKGROUND

International Publication Number WO 2012/103629 A1, AnaerobicFermentation to Produce Biogas, describes a process and apparatus forthe anaerobic digestion of organic wastes, preferably to also produce auseful biogas. The waste may have a total solids (TS) concentration of6% or less while a digester is operated at a higher solidsconcentration, for example with a feed TS concentration of 8-12%. One ormore separation stages downstream of the digester separate activebacteria and undigested organics from the digestate, and returnseparated matter to the digester. Optionally, a feed thickeningapparatus and step may be provided upstream of the digester. Theupstream thickener and recycle from the downstream separation stages areoperated such that the TS of the combined inputs to the digester iswithin a desired range.

Introduction

In a wastewater treatment system and process, feed water is processed inan anaerobic digester. A solid-liquid separation device, for example asludge screw thickener (SST), treats digestate from the digester in arecirculation loop. The solids portion is returned to the digester toincrease the solids retention time and the total suspended solids (TSS)concentration in the digester. Some solids are wasted from the digesterto maintain a target solids retention time (SRT) or TSS in the digester.A liquid portion of the digestate, which controls the hydraulicretention time (HRT), is removed from the anaerobic digester through thesolid-liquid separation device. Optionally, the liquid portion may betreated further. The flow rate to the solid-liquid separation device ispreferably greater than the influent flow rate. The solid-liquidseparation device may receive digestate at a TSS concentration of 4% ormore and return a solids portion having a TSS concentration of over 10%.The feed water is preferably one or more industrial waste streams havinga chemical oxygen demand (COD) concentration of about 20,000 to 50,000mg/L and a TSS concentration of about 1-5%. The organic loading rate maybe about 10-12 kg/COD/m3/day.

Without intending to be limited by theory, the system and process arebelieved to be effective at treating industrial wastewater due to acombination of factors. The soluble organic carbon is reduced quicklywhile the retention time of particulate organic carbon is increased bythe solid-liquid separation device. The solids retention time is alsosufficient to remove fats, oils and grease (FOG) which, if notimmediately digested, return with the solids portion. Furthermore,active anaerobic bacteria are returned to the digester with the solidsportion. Returning bacteria to the digester, in combination withinfluent containing a high concentration of soluble COD, allows thedigester to have a high percentage of its solids as living bacteria. Thedigester can therefore operate at a high organic loading rate whilestill maintaining an acceptable food to microorganism ratio anddigesting a high percentage of the influent COD. The rapid and extensivedigestion is balanced by the solid-liquid separation device receivingdigestate at an already significant initial TSS concentration and a flowrate greater than the influent flow rate, returning a solids portionthickened to at least twice the initial solids concentration, and losingless than 5% of the solids fed to it in the liquid portion.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic process flow diagram for a wastewater treatmentsystem.

FIG. 2 is a schematic process flow diagram for a first further treatmentoption for the system of FIG. 1.

FIG. 3 is a schematic process flow diagram for a second furthertreatment option for the system of FIG. 1.

FIG. 4 is a schematic process flow diagram for a first further treatmentoption for the system of FIG. 1.

FIG. 5 shows a schematic cross sectional view of a screw press with ascreening section.

FIG. 6 shows a portion of the screening section of FIG. 1 with optionalscreening section mounts and restraints and optional drive mechanismsfor rotating the screening body.

DETAILED DESCRIPTION

FIG. 1 shows a system 10 for treating wastewater. The system isprimarily intended for treating industrial wastewater. Industrialwastewater tends to have a high chemical oxygen demand (COD), forexample 20,000 to 50,000 ppm. A significant portion, for example 50% ormore, of the COD may be soluble. The industrial wastewater might, ormight not, also have a medium (typically 1-2%) or high (3-5%) totalsuspended solids (TSS). Industrial wastewater may also have a materialconcentration of fats, oil or grease (FOG), for example 300 to 4,000mg/L or about 5 to 15% of the volatile solids (VS) in the industrialwastewater. Optionally, the system may treat a combination of anindustrial waste solids stream and an industrial wastewater stream, thecombination having characteristics as described above. The waste streamsare treated primarily by anaerobic digestion. Optionally, effluent fromthe anaerobic digester may be subsequently treated with one or morephysical, chemical or biological treatments.

In the system 10, influent A is treated in an anaerobic digester 14.Influent A preferably has characteristics described in the paragraphabove. Influent A may be a single stream or a composite, whether mixedor not, of two or more waste streams. Digestate B, alternatively calledeffluent or sludge, is sent to a solid-liquid separation unit 15. Thesolid-liquid separation unit 15 separates the digestate B into adigestate solids portion C and a digestate liquid portion F. Thedigestate solids portion C is returned to the anaerobic digester 14. Thedigestate solids portion C includes suspended solids such as bacteria,non-digested solids and FOG. Returning the digestate solids portion C tothe anaerobic digester 14 while removing some or all of the digestateliquid portion F results in the solids retention time (SRT) of theanaerobic digester 14 being higher than its hydraulic retention time(HRT). The total suspended solids (TSS) content of the anaerobicdigester 14 is also higher compared to a digester with the same HRTwithout digestate solids portion C recycle.

The anaerobic digester 14 may be one of numerous vessels andconfigurations including but not limited to continuous stirred tankreactor (CSTR), plug flow reactor (PFR), suspended growth reactor,attached growth reactor or a combination thereof. CSTRs, which includemixed tank reactors generally, are preferred since they are reliable andinexpensive. However, conventional CSTRs with a 20-30 day HRT and SRTare often inefficient due to a low concentration of active biomass. Lowconcentrations of active biomass require low organic loading rates andlarge tank volumes due to the need to avoid bacteria washout and providean adequate food to micro-organism (F/M) ratio. A low solidsconcentration in conventional digesters also limits the suspended solidsconcentration that can be tolerated in the influent A. However, thesolid-liquid separation unit 15 decouples the SRT from the HRT. Theresult is an anaerobic digester 14 capable of handling high loadingrates of soluble organics and medium loading rates of particulateorganics while providing sufficient solids residence time forparticulate solids and FOG to undergo hydrolysis, assimilation anddegradation. Additionally, the solid-liquid separation unit 15 minimizesthe risk of bacteria washout.

The digestate liquid portion F is not returned to the anaerobic digester14 and may be disposed of. For example, the digestate liquid portion Fmay be sent to a sewer if permitted in a particular location.Optionally, the digestate liquid portion F may be treated further by oneor more chemical, physical or biological treatments steps in a polishingunit 17. Some specific polishing options will be discussed below inrelation to FIGS. 2 to 4. The polishing unit 17 produces an effluent H.The effluent H is preferably at least closer than digestate liquidportion F to meeting a desired standard for re-use or disposal of theeffluent H. In some cases, the polishing unit 17 may also produce asolids stream or sludge, which is referred to generically in FIG. 1 aspolishing solids portion J. Preferably, the polishing solids portion Jis returned to the anaerobic digester 14. Alternatively, the polishingsolids portion J may be sent to a dewatering unit 20, or treated ordisposed of by other methods.

A digester sludge D is also removed from the anaerobic digester 14. Thedigester sludge D may be removed periodically, for example once a day,or continuously. The digester sludge D may be removed from a separateoutlet or from a branch of a line carrying the digestate B. The digestersludge D is sent to a dewatering unit 20, for example a screw press, acentrifuge, a belt filter press or a plate and frame press. Thedewatering unit 20 produces a dewatering liquid portion L and adewatering solids portion K. The dewatering liquid portion L may bedisposed of or treated in the system 10 or another facility. Forexample, the dewatering liquid portion L may optionally be treated inthe polishing unit 17, if any. The dewatering solids portion K isremoved from the system 10.

The dewatering solids portion K is typically dried further to produce acake that is taken, for example, to a landfill, to a compostingfacility, or to an incinerator. The dewatering solids portion K containssome solids from the influent A that are inert or volatile but difficultto digest in the anaerobic digester 14 in any reasonable SRT, and alsoexcess biomass produced in the anaerobic digester 14. Removing thedigester sludge D, and in particular the dewatering liquid portion K,limits the concentration of inert or difficult to digest solids andbiomass in the anaerobic digester 14. Removing the digester sludge Dalso provides the primary control of solids retention time (SRT) andsolids concentration in the anaerobic digester 14 since solids in thedigestate B are mostly recycled to the anaerobic digester 14. The amountof digester sludge D removed is such that the solids removed balancebackwash yield and non-volatile suspended solids entering the anaerobicdigester 14. The bacterial yield is typically about 8 to 10% of the CODremoved in the anaerobic digester 14.

Optionally, a side stream from the digestate solids portion C may beremoved from the system 10 in place of, or as a supplement to, removingdigester sludge D. In this case, the flow of digestate B to thesolid-liquid separation unit 15 is increased by the amount of digestateB that would have been removed in digester sludge D. The solid-liquidseparation unit 15 handles a larger flow than when digester sludge D isremoved directly from the anaerobic digester 14. The digestate solidsportion C is difficult to mix with a polymer, which is required for sometypes of dewatering unit 20 such as a centrifuge. However, a sidestreamfrom the digestate solids portion C may be dewatered in a plate andframe press, for example, without adding a polymer. Further, although itis usually desirable to remove solids in the form of a dry cake, in somecases waste solids may be used without extensive dewatering, for exampleby application to agricultural land. In these cases, dewatering unit 20may be omitted and solids can be removed from the system 10 throughdigester sludge D, a sidestream from the digestate solids portion C, orfrom both.

The polishing solids portion J, if any, may also be removed from thesystem 10. However, the polishing solids portion J is typically a smallstream with a low concentration of anaerobic microorganisms, and solidsthat are mostly digestible. Accordingly, removing solids with thepolishing solids portion J might be accounted for but is not likely toreplace removing the dewatering solids portion K. The polishing solidsportion J is preferably returned to the anaerobic digester 14 since itincreases biogas production and reduces the amount of solids to bedisposed of.

Biogas G is produced in the anaerobic digester 14. The biogas G may beflared but it is preferably sent to a biogas processing unit 18 toproduce one or more of gas products M, heat N or power O. The biogasprocessing unit 18 may include one or more treatment units to upgradethe biogas G. For example, the biogas G may be treated to remove watervapour, particulates, ammonia or carbon dioxide. The biogas G may beupgraded further for injection into a natural gas pipeline, or otherwiseas a replacement for natural gas. Alternatively, the biogas G may beburned on site to create heat, electricity or both. For example, thebiogas G may be burned in a turbine of a combined heat and power unit.Heat produced from the biogas G may be used in the system 10, forexample to heat the anaerobic digester 14 or to help dry the dewateringsolids portion K to a cake. Electricity produced from the biogas G maybe used in the system, for example to power pumps or mixers in theanaerobic digester 14.

The anaerobic digester 14 typically comprises one or more tanks, inseries or in parallel or both, with mixers. Mixing in the digester tankis affected by the solids content and viscosity of the digestate in thetank. For example, increasing the solids content from 2.5% to 5% willusually result in a ten-fold increase in viscosity. The anaerobicdigester 14 is preferably operated at a total solids (TS) content of 4to 7%, or 5 to 6%. Operating at a 5 to 6% TS content results in aviscosity of 5 to 7 Pa·s (5,000 to 7,000 cP). Digestate with thisviscosity cannot be properly mixed with common mixing systems such astop entry, jet, draft tube, linear or gas mixers. The high viscosity andoperating temperature (about 38 degrees C. if mesophilic or about 55degrees C. if thermophilic) also makes electric submersible mixersinadequate as the electric motors tend to overheat. Installingelectrical equipment inside a digester tank may also create risks ofexplosion associated with biogas in the headspace.

Mixing is preferably done with high torque, low speed submersiblemixers. UTS Products GmbH in Lippetal, Germany manufactures high solidscontent submersible mixers driven by a hydraulic motor that aretypically used for agricultural or industrial solids digesters. Thesemixers are controlled through service boxes. The service boxes have aretractable skirt designed to isolate the service box from the digestertank headspace. This allows safe mixer servicing without the need toempty the digester tank or stop operation. The service boxes areinstalled in the digester cover, one on top of each mixer column guide,to access the mixers for service and to enable repositioning or removalof the mixers. Fixed digester covers or double membrane covers arepreferred when using the service boxes. Each mixer uses a 22 kW externalhydraulic power unit and circulates biodegradable hydraulic oil. Ifleaks occur inside the digester then the bacteria can degrade thenon-toxic leaked oil.

Usually two or more mixers are needed per digester tank, depending onthe digester tank dimensions. The mixers are located near the tankperimeter and directed to create a rotational movement of the digestateand also to reintroduce floating layers or crusts back into the bulk ofthe digestate. The UTS hydraulic power units have the ability to driveup to 5 mixers. The mixers have automatic rotation reversal if a suddentorque increase is detected, which could be attributed to rags or anaccumulation of hair or other fibers in the mixer blades. The verticalsupporting columns of the mixers allow flexibility in directing andpositioning the mixers so that the mixing energy can be effectivelyused.

Mixing is done intermittently, usually 20% of the time. Typical mixingintervals are 10 minutes ON and 40 minutes OFF, although other cyclescan be used. Continuous mixing is unnecessary and more energy consuming.Further, propionate inhibition can occur with constant mixing in bothmesophilic and thermophilic digesters.

The anaerobic digester 14 contains microorganisms, primarily bacteria,to digest the influent A to produce biogas G and digestate B. Theanaerobic digester 14 is operated at about a 4 to 7%, preferably 5-6%,TSS content. The digestate B has about the same solids content as thetank of the anaerobic digester 14. Preferred HRT and SRT depend on thedegradability of the influent A, SRT is typically over 25 days. The HRTcan be as short as 3 hours for mostly soluble COD or up to 3 days forinfluent A with high TSS content.

The anaerobic digester 14 is preferably heated to maintain thetemperature in a mesophilic or thermophilic range. Heating may be doneusing a recirculating sludge loop from the anaerobic digester, into aheat exchanger, and back to the anaerobic digester 14. Externaltube-in-tube or double spiral heat exchangers may be used. Due to theviscosity of the digestate, tube-in-tube exchangers require largepassages to reduce head losses and facilitate cleaning, and also requireinternal static flow deflectors in the sludge side to promote sludgeturbulence and increase heat transfer efficiency. This type oftube-in-tube exchanger is available from a few manufacturers and ispreferred over spiral exchangers, which usually are more costly and havehigher friction losses. The recirculating loop uses a positivedisplacement pump operated with continuous or intermittent pumping. Therecirculating loop preferably has an in line grinder to reduce the riskof a heat exchanger plugging with rags or fibers.

The digestate may be pumped continuously through the heat exchangerwhile hot water is pumped through the exchanger as needed to maintainthe desired temperature. In this case, temperature control is done onthe hot water side of the heat exchanger, automatically introducing newhot water when needed by means of a temperature control valve. Hot waterfrom a boiler or waste heat from burning biogas G may be used to heatthe digestate. A heat pump may also be used to recover some heat fromthe digestate liquid portion F if it is not useful for a polishingtreatment.

The solid-liquid separation unit 15 operates at about at a TSS recoveryof 90% or more, preferably 95% or more. Since the digestate solidsportion C returned to the anaerobic digester 14 adds to the influentflow and must be passed back through the solid liquid separation unit15, it is beneficial to maximize the thickening ratio of the solidliquid separation unit 15. Maximizing the thickening ration of thedigestate solids portion C also reduces the rate of polymer consumptionin the solid liquid separation unit 15. Polymer consumption is afunction of the solids mass loading (flow rate multiplied by solidsconcentration) that the solid liquid separation unit 15 receives. Flowrate of digestate, and therefore solids mass loading, are reduced asthickening ratio increases. Increasing the thickening ratio also reducesthe size of the solid liquid separation unit 15, which is also afunction of solids mass loading. However, it is not desirable to have tomove the digestate solids portion C on a conveyor as a cake.Accordingly, the digestate solids portion C is preferably thickened tonear the highest concentration that may be pumped back to the anaerobicdigester 14.

The TSS of the digestate solids portion C may be 2 to 3 times or morethan the TS of the digestate B. The TSS of the digestate solids portionC in the system 10 may be over 10%, preferably 12 to 14%. For example,digestate B at 4 wt % solids may be thickened to produce a digestatesolids portion C at 12 wt %. The rate of flow of digestate B to thesolid-liquid separation unit 15 is preferably at least as much as theflow rate (Q) of the influent A. Preferably, the rate of flow ofdigestate B to the solid-liquid separation unit 15 is at least 110% of Qor at least 120% of Q.

The solid-liquid separation unit 15 may be a drum, disc, or screwthickener (alternatively called a screw press, a sludge screw thickeneror press or a rotary screw thickener or press). Other devices, such asclarifiers, dissolved or cavitation air flotation units, centrifugethickeners, and membranes, are not useful for producing a digestatesolids portion C with over 6% solids.

The solid-liquid separation unit 15 may have a screen or mesh having anopening size in the range of about 200 to 500 microns. Digestate B ispumped, for example with a positive displacement pump, from theanaerobic digester 14 to the solid-liquid separation unit 15. An in-linegrinder can be installed in the pipe feeding the solid-liquid separationunit 15 in cases where the influent A contains fibers or large pieces.Digestate solids portion C, for example at 12% to 14% solids, is sentback to the anaerobic digester 14. For example, digestate solids portionC may drop from the solid-liquid separation unit 15 into a hopper-fedpositive displacement pump such as a rotary lobe or progressive cavitypump.

Solids recovery is enhanced by adding a polymer to the digestate B tothe solid-liquid separation unit 15. For example, Polymer E, typicallyin the form of a dilute solution, may be injected upstream of thesolid-liquid separation unit 15. A high shear static mixer or mixingvalve is used to disperse the polymer E into the digestate B.Flocculation is done in the pipe between the pump and the solid-liquidseparation unit 15 as the digestate B and polymer E approach thesolid-liquid separation unit 15. Typical polymer doses range from 4 to 6kg per ton of solids.

A preferred solid liquid separation unit 15 is an enclosed rotary screwthickener with an internal screw, designed to receive sludge with highinitial solids content (3 to 7%). A suitable screw thickener isdescribed in U.S. Provisional Patent Application 61/636,000 which isincorporated by reference. Such a screw thickener (alternatively calleda screw press) has an auger shaft within a screening section, the augershaft having an increased diameter towards an outlet end of the screwpress. The screening section has openings sized to remove floc fromsludge (for example digestate B), for example 200 to 500 microns orwedgewire with a slot opening in the range from about 0.25 mm to about0.75 mm. The screening section may be selectively fixed or allowed torotate. A sprayer system can be used to spray water against the outsideof the screening section. The screening section is cleaned periodicallyby spraying water against the screening section while rotating it. Thescreening section is enclosed.

A screw thickener generally as described in U.S. Provisional PatentApplication 61/636,000 is available commercially from UTS. Thisthickener is enclosed, uses low energy, has low polymer demand andachieves high solids capture. It is designed to receive up to 5 to 7%solids influent and to produce a 12 to 14% solids digestate solidsportion C. Such a screw press will be described further below withreference to FIGS. 5 and 6.

FIG. 5 shows a screw press 110 having an inlet end 106 and an outlet end108. The screw press 110 has a cylindrical screening body 112 and aframe 116. The screening body 112 has a non-porous inlet section 120, aporous screening section 122 and a non-porous outlet section 124. Theframe 116 similarly has an inlet portion 126, a central portion 128 andan outlet portion 130. The inlet portion 126 of the frame 116 may sharecomponents with or support the inlet section 120 of the screening body112. The central portion 128 of the frame 116 at least partiallysurrounds the screening section 122 of the screening body 112. Theoutlet portion 130 of the frame 116 may share components with or supportthe outlet section 124 of the screening body 112.

The screw press 110 also has an auger 114 and a drive mechanism 118. Theauger 114 is located within the screening body 112 and is supported bythe frame 116. In particular, the auger 114 and screening body 112 areconcentric about a central axis 132 of the screw press 110. One end of ashaft 160 of the auger 114 is supported through a bearing (not shown) ona stationary post 134 attached to the outlet portion 130 of the frame116. The other end of the auger shaft 160 of the auger 114 is supportedon a drive shaft 136 of the drive mechanism 118. A blade 158 of theauger 114 is attached in a spiral around the auger shaft 160 and extendstowards, or optionally touches, the inside of the screening body 112.The drive mechanism 118 is attached to the inlet portion 126 of theframe 116 and comprises a motor 138 and a gearbox 140. The frame 116 issupported on the ground through a frame 141.

The inlet portion 126 of the frame 116 has an inlet 142 to receive afeed mixture 144 such as digestate B. The central portion 128 of theframe 116 has a liquid outlet 146 to discharge a liquid fraction 148,such as digestate liquid portion F, of the feed mixture 144. The outletportion 130 of the frame 116 has a solids outlet 150 to discharge asolids fraction 152, such as digestate solids portion C, of the feedmixture 144. The liquid fraction 148 may have some solids remaining init but at a reduced solids concentration relative to the feed mixture144. The solids fraction 152 may have some liquid in it but at a highersolids concentration than the feed mixture 144.

In operation, the feed mixture 144 is pumped into the inlet 142 at aninitial pressure. The drive mechanism 118 rotates the auger 114 causingthe blade 158 to convey the feed mixture 144 along the screening body112. The auger 114 preferably also increases the pressure of the feedmixture 144. Liquid and fine solids in the feed mixture 144 are forcedthrough the screening body 112. These liquids and fine solids arecollected in the central portion 128 of the frame 116 and dischargedthrough liquid outlet 146. The remainder of the feed mixture 44 exitsthe screening body 112 after passing by a counter pressure cone 160.Counter pressure cone 160 is biased towards the screening body 112 by abiasing mechanism 162 such as a set of springs or a pneumatic cylinder.The solids fraction 152 drops from the end of the screening body 112 andis discharged from the frame 116 through the solids outlet 150.

The internal volume of the screening section 122 preferably decreasestowards the outlet end 108 of the screw press 110. This helps maintainpressure in the screening section 122 even though the liquids fraction148 is removed from the feed mixture 144. A decreasing volume may beobtained by reducing the diameter of the screening section 122.Alternatively, the pitch of the blades 158 may be decreased towards theoutlet end 8 of the screw press. Both of these methods, however, preventthe use of a blade 158 having a uniform outer diameter and pitch, whichis more easily manufactured to a tight fit with the screening section122. In the screw press 110 of FIG. 5, a decreasing internal volume isprovided by increasing the diameter of at least a portion of the shaft160 towards the outlet end 108 of the screw press 110.

The screening section 122 of the screening body 112 has openings of asize and shape adapted to provide a selected degree of separation. Forexample, the screening section 122 may have smaller openings suited toseparating flocculated solids from the digestate B. For example, thescreening section 122 may be made from wedgewire with a slot opening inthe range from about 0.25 mm to about 0.75 mm. Alternatively, thescreening section 122 may be made of other materials or have openings inthe range of about 200 to 500 microns.

Although any screw press may benefit from having a convenient cleaningmethod, cleaning is required more frequently when the screening section122 has small openings. In particular, when the screw press 110 is usedto thicken sludge, dismantling the screw press 110 for cleaning isundesirable. The screw press 110 is fitted with a sprayer system 170 toallow cleaning by spraying water against the outside of the screeningsection 122. When cleaning is required, water is pumped through amanifold 172 to a series of sprayer heads 174 located inside of thecentral portion 128 of the frame 116. The screw press 110 of FIG. 5 hasone manifold 172, but there may be multiple manifolds 172 spaced aroundthe circumference of the screening body 112.

The water sprayed against the screening section 122 moistens and breaksup accumulations of solids caught in the openings of the screeningsection. Some of the water may also force its way through the openingsof the screening section 122 in a reverse direction. The water ispreferably heated. In order to assist the water in cleaning thescreening section 122, the supply of feed mixture 144 may be stoppedwhile the auger 114 continues to rotate for a period of time before thewater is sprayed. This reduces the volume or pressure, or both, of thefeed mixture 144 inside the screening section 122.

Referring to FIG. 6, the screening section 122 may be made up of screenpanels 182 having openings 184. In the construction shown in FIG. 6, thescreening section 122 is made up of segments 184 each having acylindrical screen panel 182 attached to a forward flange 186 and arearward flange 188. The screening section is built up by attaching therearward flange 188 of one segment 184 to the forward flange 186 ofanother segment 184, optionally by way of fasteners 190. Other methodsof constructing a screening section 122 may also be used.

Referring back to FIG. 5, a forward flange 186 at one end of thescreening section 122 is held within a receiver 194 attached to theframe 116 through the inlet section 120 of the screening body 112.Alternatively, the receiver 194 may be attached directly to the frame116. The receiver 194 contains a bearing, such as a brass or plasticring or a race of ball bearings, and allows rotation of the screeningsection 122. Similarly, a receiver 194 is attached to the frame 116 andholds, but allows rotation of, the other end of the screening section122. This second receiver 194 may hold a rearward flange 188 or asupplementary flange 192 fastened to a rearward flange 188. In this way,at least the screening section 122 of the screening body 112 is allowedto rotate about the central axis 132. Optionally, one or more non-porousparts of the screening body 112 may also be allowed to rotate.Optionally, intermediate bearing mechanisms 196 may be provided tosupport, but allow rotation of, the screening section 122.

Referring back to FIG. 6, each bearing mechanism 196 has a roller 198supported through a post 200 by the frame 116. The roller 198 spins onan axle 202 supported by the post 200. Three or more bearing mechanisms196 may be spaced around the circumference of the frame 116 at eachlongitudinal position shown in FIG. 5 to better support and center thescreening section 122. A lever 204 supported on the end of an axle 202is attached to an actuator 206. When the actuator 206 is moved to theleft, the lever 204 bears against a forward flange 186. The lever 204may apply friction to reduce the speed of rotation of the screeningsection 122 or to stop the screening section 122 from rotating. Movingthe actuator 206 to the right lessens or removes the friction. Movingthe actuator 206 even further to the right moves the lever 204 to theoutside of the forward flange 186 so that the screening section 122 canbe pulled out of, or inserted into, the frame 116.

An alternative mechanism for supporting or facilitating a desiredrotation of the screening section 122 is shown at the top of FIG. 6. Inthis alternative, rearward flanges 188 are made in the form of ringgears. A gear shaft 210 supported by the frame 116 is fitted with gears212 that engage the rearward flanges 188. The gear shaft 210 may besupported by the frame 116 at intermediate positions to allow the gears212 to help support or center the screening section 122. Alternativelyor additionally, the gear shaft 210 may be attached to a brake such thatthe gear shaft 210 can be used to stop or slow the rotation of thescreening section 122. Alternatively or additionally, the gear shaft 210may be connected to the gearbox 140 of the drive mechanism 118, or to aseparate drive mechanism, so that the gear shaft 210 can be used todrive the rotation of the screening section 122.

When rotation of the screening section 122 is not restrained, thescreening section 122 will tend to rotated with the auger 114 due tofriction between the auger 114 and the screening section 122.Optionally, the screening section 122 may be forced to rotate with theauger 114 by actuating a releasable connection (not shown) between theauger 114 and the screening section 122. Alternatively, the screeningsection 122 may be driven by the drive mechanism 118 without applyingforce through the auger 114, for example by use of the gear shaft 210 ofFIG. 6. In all of these examples, the motor 138 is used, directly orindirectly, to rotate the screening section 122. A separate motor mayalso be used to rotate the screening section 122. In some of theexamples above, the screening section 122 may be rotated when the auger114 is not rotating or at a different speed than the auger 114. In someother examples, such as applying some friction to the screening section122 while the auger 114 is rotating, the screening section 122 can onlybe rotated while the auger 114 is rotated but the screening section 122may rotate at a different speed than the auger 114.

The screening section 122 is preferably rotated while water is sprayedagainst it during the cleaning process described above. Rotating thescreening section 122 allows each part of the screening section to passunder a line of sprayer heads 174. Multiple manifolds 172 are notrequired. Rotating the screening section 122 also allows each part ofthe screening section 122 to be placed at or near the top of thescreening section 122 so that gravity may assist in the cleaning.Rotating the screening section 122 also avoids having the bottom of thescreening section 122 accumulate solids that fall from upper parts ofthe screening section 122 during cleaning. Optionally, the screeningsection 122 may be stopped and the auger 114 may be rotated for a periodof time during or after the cleaning procedure to convey backwashedsolids out of the screening section 122. Alternatively, the auger 114may be rotated at a faster speed than the screening section 122 duringthe cleaning process to convey material released during cleaning out ofthe screening section 122.

After the cleaning procedure, the screening section 122 is restrainedfrom rotating, the auger 114 resumes or continues normal rotation, andthe supply of feed mixture 144 is restarted. The screening section 122is typically prevented from rotating during operation of the filterpress 110 except when cleaning the screening section 122.

A rotary drum thickener might also be used, but is less desirable. Inthese, a screen drum rotates with internal welded flights moving thesludge forward as it drains. Some commercially available models canproduce a solids content of 8 to 10%, but a solids content in thedigestate solids portion C of over 10%, or over 12% is preferred. Rotarydrum thickeners are also typically limited to a solids concentration ofthe digestate D of 3% or less which would prevent operating theanaerobic digester 14 at the preferred solids content of 4 to 7%. Rotarydrum thickeners are also less efficient than rotary screw thickeners andare rarely enclosed. An enclosed device is preferred for odor control,since ammonia and hydrogen sulfide would otherwise escape to theatmosphere. Thickening centrifuges can also be used, but these are morecostly than screw thickeners and require more energy to operate.

The anaerobic digester 14 may operate, for example, at 5% solids with areturn of digestate solids portion C thickened to 12-14% solids. Sincethe solid-liquid separation device 15 returns most of the livingbacteria, and the soluble fraction of the influent A promotes rapidbacteria growth, a significant portion of the solids in the anaerobicdigester 14 is living bacteria. The F/M ratio can therefore be highrelative to standards based on the digester solids content. Theanaerobic digester 14 may operate with an organic loading rate (OLR) ofabout 10-12 kg COD/m3-day. The loading rate of particulate COD can be upto 7 kg COD/m3-d.

Ammonia is produced in the anaerobic digester from organic nitrogenbeing mineralized to ammonia. Alkalinity may need to be added to theanaerobic digester 14 to allow operation with ammonia in the digestate.

Digestate liquid portion F may have various contaminants, in particularTSS, COD and ammonia. The TSS concentration is typically in the range of1,000 to 3,000 mg/L. If the digestate liquid portion F cannot bedischarged, for example to a sewer, it may be treated first in thepolishing unit 17. Some examples of polishing units 17 are shown inFIGS. 2 to 4.

In FIG. 2, a first polishing unit 17A has a dissolved air flotation unit22. The dissolved air flotation unit 22 is effective primarily to reducethe suspended solids concentration of the digestate liquid portion F.Chemicals 24, for example one or more of a coagulant and a polymer, maybe added to the digestate liquid portion F. The chemicals 24 help createa floc that floats with micro-bubbles in the dissolved air flotationunit. Solids are recovered in the float and may be returned to theanaerobic digester 14 as polishing solids portion J. The effluent H leftafter the float is removed still contains COD and ammonia but issuitable for discharge to a sewer in many places. A cavitation airflotation device may be used with chemicals 24 in a similar way.Alternatively, tubular cross flow membranes may also be used to removesuspended solids from the digestate liquid portion F without the use ofchemicals 24.

In FIG. 3, a second polishing unit 17B provides biological nutrientremoval. If there is a requirement for high quality effluent H fordischarge to a water course or for reuse, then it is usually necessaryto remove nitrogen, COD and suspended solids with aerobic biologicaltreatment. Aerobic treatment could be by a conventional activated sludgesystem with nitrogen removal, a membrane bioreactor, or an attachedgrowth system. The second polishing unit 17B shown is a membranebioreactor having an anoxic tank 26, an aerobic tank 28 and an aerobicmembrane tank 30. Alternatively, a clarifier could be substituted forthe aerobic membrane tank 30 to create a conventional activated sludgeprocess, optionally with a downstream tertiary filter. Return activatedsludge (RAS) is recycled from the separation tank 30 to the anoxic tank26. Waste activated sludge (WAS) is taken from the separation tank 30 toa rotary drum thickener 32. Filtrate R from the rotary drum thickener 32is sent to the anoxic tank 26. The recycle through the anoxic tank 26and aerobic tank 28 removes nitrogen by way of anitrification—denitrification process. Thickened WAS from the rotarydrum thickener 32 may be returned to the anaerobic digester 14 aspolishing solids portion J.

In FIG. 4, a third polishing unit 17C comprises an anaerobic membranebioreactor. Digestate liquid portion F is received in a mixing tank 34and then passes to a membrane tank 36. Both tanks 34, 36 are covered. Abiogas compressor 38 recirculates biogas to a membrane unit in themembrane tank 36. Return sludge S flows from the membrane tank 36 to themixing tank 34. The mixing tank 34 allows the membrane tank to operatewithout over concentrating. The flow rate of return sludge S may beabout 3 or 4 times the flow rate of the digestate liquid portion F. Forexample, the membrane tank might operate at a mixed liquor suspendedsolids (MLSS) concentration of about 8,000 mg/L while the mixing tankhas an MLSS of about 6,000 mg/L. A portion of the return sludge S isremoved as waste sludge and returned to the anaerobic digester 14 aspolishing solids portion J. The third polishing unit 17C removescolloidal solids due to the membrane filter. However, because an aerobicbiomass is not generated, the membrane surface area and aeration energyrequired are much lower than for the second polishing unit 17B. Removingcolloidal solids provides increased reduction in TSS and measured CODrelative to the first polishing unit 17A but the ammonia concentrationis not significantly reduced. Optionally, an ammonia stripper may beadded to the third polishing unit 17C to recover ammonia for reuse.

In comparison, industrial wastewater with a (COD) of 20,000 to 50,000ppm and up to 1.5% solids could be treated with a DAF unit followed byan anaerobic granular sludge reactor. The DAF unit thickens thewastewater to about 4% solids for feeding to the granular reactor.However, raw industrial wastewater at 4% solids is not typically fed togranular reactors. Raw wastewater at more than about 2% solids tends tohave larger solid particles and FOG which are not tolerated by granularreactors and would need to be removed in upstream processes. Removedsolids and grease have to be hauled off from industries and do notcontribute to the biogas and energy generated. Accordingly, the system10 is able to receive and treat a wider range of industrial wastes. Thesystem 10 also avoids some operational difficulties with granular sludgesystems, such as slow start up and seeding procedures, and avoids therisk of washing out suspended granular biomass.

In a modelling exercise, the system 10 was compared to a system having aDAF unit followed by an expanded granular sludge bed (EGSB) reactor. Theinfluent was assumed to be a 0.5 MGD flow of industrial wastewaterhaving 22,800 mg/L tCOD; 10,800 mg/L sCOD; and 10,000 mg/L (1%) TSS.Capital costs for the systems were similar, although the system 10 wasabout 5% less expensive. The operating cost of the system 10 was over40% less than the operating cost of the DAF-EGSB system. The costsavings were due in large part to a reduction in solids disposal costs.Although not factored into the operating costs, the system 10 alsoproduced more than twice as much biogas as the DAF-EGSB system. Thebiogas could be sold or used within the system 10 to further reduce theoperating cost and energy consumption of the system 10.

In the description above, the terms solids portion and liquid portionindicate the higher solids content and lower solids content,respectively, of two streams produced from a solid-liquid separationdevice. The solids portion still contains some liquid, and the liquidportion still contains some solids. Depending on the particularsolid-liquid separation device used, the solids portion might be calledscreenings, cake, retentate, reject, thickened solids, sludge, bottomsor by other terms. The liquid portion might be called effluent,permeate, filtrate, centrate or by other terms.

The word “digestate” is sometimes used in the art to refer specificallyto the liquid portion of an effluent stream taken from an anaerobicdigester. In this specification, however, digestate B is used to referto a stream drawn from a digester before it is separated. Parts of thisspecification also relate to anaerobic digesters in the form of mixedtanks or CSTRs. In these cases, the word digestate is also used to referto the entire mixed contents of the tank and digestate B withdrawn fromthe tank is similar in composition to digestate in the tank.

Unless stated otherwise or apparent form the context, solids contents orconcentrations mentioned in this specification are total suspendedsolids (TSS). In digestate, the total solids (TS) is approximately thesame as dried solids (DS) and roughly 10% higher than total suspendedsolids (TSS). For example, a 5% DS digestate may have 46,000 mg/L of TSSand 4000 mg/L total dissolved solids (TDS). Accordingly, solids contentsor concentrations of one type given in this specification can generallybe substituted with TS or DS concentrations without causing a materialdifference in the process.

The descriptions of processes and apparatus above are to provide atleast one example of an embodiment within each claim but not to limit ordefine any claim. However, it is possible that a particular process orapparatus described above is not within a specific claim. Processparameters are given as examples of how a system may be operated and arenot meant to limit a claim unless explicitly recited in a claim. Otherprocesses for similar applications might operate at parameters withinranges that are 50% or 100% larger in both directions than the parameterranges described above, or within a 50% or 100% variation from a singleparameter described above. If one or more elements or steps describedabove are used to treat other wastes or under other conditions, then oneor more process ranges described above might not be suitable and wouldbe substituted with other appropriate parameters. Various sub sets ofthe unit processes described in relation to system 10 can be used inother treatment plants. Various sub sets of unit processes in the system10 described above may also be combined in ways other than thosedescribed to produce different systems. Words such as “may”,“preferable” or “typical”, or variations of them in the descriptionabove, indicate that a process step or apparatus element is possible,preferable or typical, according to the word used, but still optionaland not necessarily part of any claimed invention unless explicitlyincluded in a claim.

1. A method for treating wastewater comprising steps of, a) feeding aninfluent at a flow rate Q to an anaerobic digester; b) feeding a mixturefrom the anaerobic digester a flow rate of Q or more to a solid liquidseparation device; c) separating the mixture into a liquid portion and asolids portion; and, d) returning at least most of the solids portion tothe anaerobic digester.
 2. The method of claim 1 wherein the solidliquid separation device is a screw thickener.
 3. The method of claim 1wherein substantially all of the solids portion is returned to theanaerobic digester.
 4. The method of claim 1 wherein the anaerobicdigester has a TSS concentration of 4 to 7%.
 5. The method of claim 1wherein the solids portion has a TSS concentration of over 10%.
 6. Themethod of claim 1 wherein the solids portion has a TSS concentration ofover 12%.
 7. The method of claim 1 wherein the solids portion has a TSSconcentration in the range of 12-14%.
 8. The method of claim 1 whereinthe organic loading rate of the anaerobic digester is at least 10kg/COD/m3/day.
 9. The method of claim 1 wherein the influent has achemical oxygen demand concentration of about 20,000 to 50,000 mg/L anda total suspended solids concentration of about 1-5%.
 10. The method ofclaim 1 wherein the solids portion contains 95% or more of the suspendedsolids in the mixture.
 11. The method of claim 1 wherein the solidsportion has a TSS concentration that is at least twice the TSSconcentration of the mixture.
 12. The method of claim 1 furthercomprising a step of treating the liquid portion in a second solidliquid separation device.
 13. The method of claim 12 wherein the secondsolid liquid separation device is selected from the group consisting ofa dissolved air flotation unit, a cavitation air flotation unit, and atubular membrane.
 14. The method of claim 12 wherein the second solidliquid separation device is a dissolved air flotation unit.
 15. Themethod of claim 1 further comprising a step of treating the liquidportion in an activated sludge reactor.
 16. The method of claim 15wherein the activated sludge reactor is a membrane bioreactor.
 17. Themethod of claim 1 further comprising a step of treating the liquidportion in an anaerobic membrane bioreactor.
 18. The method of claim 1further comprising a step of adding a polymer to the mixture.
 19. Asystem for treating an influent comprising, a) an anaerobic digester;and, b) a re-circulation loop from the anaerobic digester to a screwpress and back to the anerobic digester.
 20. The system of claim 19wherein the screw press has a screening section that may be selectivelyfixed or allowed to rotate.
 21. The system of claim 19 wherein the screwpress has a screening section with openings in the range of 200 to 500microns or made from wedgewire with a slot opening in the range fromabout 0.25 mm to about 0.75 mm.
 22. The system of claim 19 wherein thescrew press has an auger shaft with an increased diameter towards theoutlet end of the screw press.
 23. The system of claim 19 wherein thescrew press has a sprayer system adapted to spray water against theoutside of a screening section.
 24. The system of claim 19 wherein thescrew press has an enclosed screening section.